Various processes are known for the production of higher linear alpha olefins (for example D. Vogt, Oligomerisation of ethylene to higher α-olefins in Applied Homogeneous Catalysis with Organometallic Compounds Ed. B. Cornils, W. A. Herrmann, 2nd Edition, Vol. 1, Ch. 2.3.1.3, page 240–253, Wiley-VCH 2002). These commercial processes afford either a Poisson or Schulz-Flory oligomer product distribution.
In order to obtain a Poisson distribution, no chain termination must take place during oligomerisation. However, in contrast, in a Schulz-Flory process, chain termination does occur and is independent from chain length. The Ni-catalysed ethylene oligomerisation step of the Shell Higher Olefins Process (SHOP) is a typical example of a Schulz-Flory process.
In a Schulz-Flory process, a wide range of oligomers are typically made in which the fraction of each olefin can be determined by calculation on the basis of the so-called K-factor. The K-factor, which is indicative of the relative proportions of the product olefins, is the molar ratio of [Cn+2]/[Cn] calculated from the slope of the graph of log [Cn mol %] versus n, where n is the number of carbon atoms in a particular product olefin. The K-factor is by definition the same for each n. By ligand variation and adjustment of reaction parameters, the K-factor can be adjusted to higher or lower values. In this way, the process can be operated to produce a product slate with an optimised economic benefit.
Since demand for the C6–C18 fraction is much higher than for the C>20 fraction, processes are geared to produce the lower carbon number olefins. However, the formation of the higher carbon number olefins is inevitable, and, without further processing, the formation of these products is detrimental to the profitability of the process. To reduce the negative impact of the higher carbon number olefins and of the low value C4 fraction, additional technology has been developed to reprocess these streams and convert them into more valuable chemicals such as internal C6–C18 olefins, as is practised in the Shell Higher Olefins Process.
However, this technology is expensive both from an investment and operational point of view and consequently adds additional cost. Therefore considerable effort is directed to keep the production of the higher carbon numbered olefins to the absolute minimum, i.e. not more than inherently associated with the Schulz-Flory K-factor.
In this regard a number of published patent applications describe catalyst systems for the polymerisation or oligomerisation of 1-olefins, in particular ethylene, which contain nitrogen-containing transition metal compounds. See, for example, the following patent applications which are incorporated herein by reference in their entirety: WO 92/12162, WO 96/27439, WO 99/12981, WO 00/50470, WO 98/27124, WO 99/02472, WO 99/50273, WO 99/51550, EP-A-1,127,987, WO 02/12151, WO 02/06192, WO 99/12981, WO 00/24788, WO 00/08034, WO 00/15646, WO 00/20427, WO 01/58874 and WO 03/000628.
In particular, recently published Shell applications WO01/58874, WO02/00339, WO02/28805 and WO 03/011876, all of which are incorporated herein by reference in their entirety, disclose novel classes of catalysts based on bis-imine pyridine iron compounds which are highly active in the oligomerisation of olefins, especially ethylene and which produce linear alpha olefins in the C6–C30 range with a Schulz-Flory distribution, said linear alpha olefins being of high purity.
There is still a need however for improving the selectivity of the linear alpha olefins in the oligomerisation processes described in the prior art.
It is known to use a co-catalyst such as an aluminium alkyl or aluminoxane in conjunction with an olefin polymerization catalyst. It has now surprisingly been found that replacement of a portion of the aluminium alkyl or aluminoxane co-catalyst with one or more particular zinc compounds, such as ZnEt2, results in an improvement in the selectivity for linear alpha-olefins in the above mentioned oligomerisation process catalysed by bisarylimine pyridine iron compounds. At the same time, the amount of unwanted by-products such as internal and branched olefins and dienes, is reduced.
Angew. Chem. Int. Ed. 2002, 41, No. 3, pages 489–491, entitled “Iron-Catalyzed Polyethylene Chain Growth on Zinc: Linear alpha-olefins with a Poisson Distribution” by George J. P. Britovsek, Steven A. Cohen, Vernon C. Gibson, Peter J. Maddox, and Martin van Meurs, discloses a chain growth process on zinc, catalysed by a bis(imino)pyridine iron catalyst. However, analysis of the polymer produced by the reaction after hydrolysis revealed a fully saturated linear alkane product, rather than an alpha-olefin product. In order to obtain an alpha-olefin this reference describes a nickel-catalyzed displacement of the grown alkyl chain, in the presence of ethylene to regenerate diethyl zinc. The distribution of alpha olefins produced is said to be a Poisson distribution rather than a Schulz-Flory distribution.